Given that runways are numbered according to compass heading, 0/36 being due north, 9 being due east, 18 being due south, and 27 being due west.

I've noticed that there's a system of assigning L, R, and C to three parallel runways.

However, in cases of four or more parallel runways (i.e. at DFW) there's a problem so one pair of runways is numbered one degree higher/lower.

Lately, though, I've noticed yet another way that this is handled -- the two longest runways are marked as parallels and the shorter is assigned a different number. For example, at PHL three runways are parallel (except for a 0.1-degree difference). The two longer runways are marked 9L/27R and 9R/27L, while the shorter runway is marked 8/26.

The proposed third runway at CLE is marked the same way as PHL's -- although it appears to be parallel to the 5/23 parallels, it is marked as 6/24.

There are ICAO recommended practices that determine how successive parallel runways are numbered. (Someone else can chase down the actual convention!)

One thing that precludes a "C" designation is the distance between it and the "L", "R" parallels. Toronto is preparing to build yet a sixth runway upon completion of its fifth runway. The north (currently) single parallel had its designation changed from 24R/06L to 23/05 as the south runways are to be designated 24 L & R/06 R & L. Eventually, the north parallels will be designated 23 L & R/05 R & L.

Those of us who use the airport a lot feel all the additional runways that have been built or are about to be built would more properly be designated as very wide, expensive, well-lit taxiways.

At Frankfurt (EDDF) they have two runway designations for the same piece of runway. The two parallel runways are 25L/R 07L/R, with a magnetic track of 250/070.
On runway 25L they made a High Approach Landing System, which offers the possibility to reduce wake turbulence separation for medium and light aircraft to the permissible radar separation minima. For this purpose they made a second threshold 1500m down runway 25L. This threshold is designated 26L, and there is also an ILS and papi for this threshold. So the approaches ILS DME Rwy 26L and ILS Rwy 25L is for the same strip of runway.
I don't know how much they use it though. I never go to Frankfurt, just saw it in the Jeppesen.